Patients often attempt to breathe spontaneously during mechanical ventilation; therefore machines have been developed that can sense this effort. When the ventilator detects changes in pressure, flow, or volume, a patient­triggered breath occurs. Pressure and flow are common patient triggering mechanisms. When a negative pressure or change in flow is detected, inspiration begins and gas flows to the patient.
To enable patient triggering, the operator must specify the sensitivity setting, also called the patient effort (or patient­triggering) control. This setting determines the pressure or flow change that will trigger the ventilator. The lower the pressure or flow change, the more sensitive the machine is to the patient's effort. For example, the ventilator is more sensitive to patient effort at a setting of -0.5cm H20 than at a setting of -1cm H20.
The sensitivity level for pressure triggering usually is set at about -1 cm H20. The operator must set the sensitivity level to fit the patient's needs. If it is not set correctly, the ventilator may not be sensitive enough to the patient's effort, and the patient will have to work too hard to trigger the breath. If the ventilator is too sensitive, it can autotrigger (i.e., the machine triggers a breath without the patient making an effort).